Avoid AC_CHECK_LIB(deflate)'s default ACTION-IF-FOUND behaviour

The other foo_devel=ok/etc tests that do separate AC_CHECK_HEADER/
AC_CHECK_LIB checks with the default AC_CHECK_LIB ACTION-IF-FOUND
always instantly fail if either header or library is missing.

However for libdeflate this is only the case for --with-libdeflate=yes.
By default or with --with-libdeflate=check, the intention is to carry on
as per --without-libdeflate when either header or library is missing.

For the case when libdeflate.h is missing but the library is found
(admittedly an odd case!), the default AC_CHECK_LIB ACTION-IF-FOUND
also defines HAVE_LIBDEFLATE -- leading to later build failures as we
(ab)use that macro to mean both HAVE_LIBDEFLATE_H and HAVE_LIBDEFLATE.

Instead use a no-op found action, and only AC_DEFINE(HAVE_LIBDEFLATE)
and add to $LIBS when *both* checks have succeeded.

configure.ac

@@ -275,9 +275,10 @@ fi
 AS_IF([test "x$with_libdeflate" != "xno"],
   [libdeflate=ok
    AC_CHECK_HEADER([libdeflate.h],[],[libdeflate='missing header'],[;])
-   AC_CHECK_LIB([deflate], [libdeflate_deflate_compress],[],[libdeflate='missing library'])
+   AC_CHECK_LIB([deflate], [libdeflate_deflate_compress],[:],[libdeflate='missing library'])
    AS_IF([test "$libdeflate" = "ok"],
     [AC_DEFINE([HAVE_LIBDEFLATE], 1, [Define if libdeflate is available.])
+     LIBS="-ldeflate $LIBS"
      private_LIBS="$private_LIBS -ldeflate"
      static_LIBS="$static_LIBS -ldeflate"],
     [AS_IF([test "x$with_libdeflate" != "xcheck"],